Added plotter_helper.integrate

Integrates a spectrum.

    Parameters
    ----------
    spectrum :
    inplace = False
        if the sepctrum should be modified in place or not

    Todo: what to do with errors?
Changed default value of inplace to True in plotter_helper.integrate.

This seems more inline with the rest of the functions, which all work inplace by default.
Merge branch 'master' of https://github.com/SpectralGroup/spectraplotpy into integrate

RESOLVED Conflicts:
	spectraplotpy/plotter_helper.py
Merge pull request #61 from ajasja/integrate

Integrate